Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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20 MAY 2015 MATERIALS PERFORMANCE MATERIAL MATTERS NACE INTERNATIONAL: VOL. 54, NO. 5 NACE INTERNATIONAL: VOL. 54, NO. 5 MAY 2015 MATERIALS PERFORMANCE by Mass Communication Specialist 1st Class Shannon E. Renfroe. An AAV assigned to Combat Assault Co., 3rd Marine Regiment, launches from the beach to amphibious dock landing ship USS Rushmore. U.S. Navy photo 20 Polyurea coatings may extend life of amphibious assault vehicles R esearchers Dr. Mike Roland and Dr. Ray Gamache with the U.S. Naval Re- search Laboratory (NR L) (Washington, DC) have discovered that some ty pes of rubber will provide corrosion protection for amphibious assault vehicles (A AVs), and potentially offer better ballistic pro- tection as well. For the U.S. Marine Corps (USMC), this is an important f inding as they prepare to extend the use of A AVs, introduced in 1972, through 2035. "What makes [A AVs] unique is they can go in water and land," says Roland, a physi- cal chemist and leader of the research proj- ect. "They give the Marine Corps a capabil- ity that no other service has." A AVs are highly mobile, tracked ar- mored amphibious vehicles that transport Marines and cargo to and through hostile territory. Ty pically, A AVs are the f irst ve- hicles to land during beach raids and as- saults. They can operate at speeds of 45 mph (72 km/h) on land and 8 to 10 knots (4 to 5 m/s) in water; carry 21 combat- loaded Marines and three crewmembers; transport 10,000 lb (4,536 kg) of cargo; and hold enough fuel to drive 300 miles (483 km) inland. 1 These vehicles feature an all-welded aluminum hull that protects crew from small arms f ire. Since the 1990s, the USMC has been bolting armor onto its A AVs. The armor is a laminate comprised of hard coated steel on the outer surface, which is good for ballistics; a rubber layer in the center; and a soft steel layer on the back. The problem is that corrosion initi- ates at cracks in the surface paint, and the armor corrodes from intense use and exposure to salt water. "You've got steel, rubber, steel—and these things are ther- mally expanding and contracting differ- ently," Roland says. "In addition to which, A AVs aren't driven like expensive Volvos; they're banging into stuff—and now you've got a way for water ingress." NR L research shows that poly ureas could better protect the armor from cor- rosion by stretching with it instead of cracking. The poly urea coatings also slow bullets and blast fragments. "They take kinetic energ y from the bullet," says Roland. "So the bullet, to keep penetrat- ing, it's meeting an increasingly resistant medium. And if it slows down enough— it always makes it to the steel plate, but it doesn't have enough velocity to get through it." Poly ureas aren't as common as con- ventional rubbers (poly urethanes), which are produced by reacting isocyanates with a range of polyols. When isocyanate is reacted with an amine, it produces urea, Roland explains. "Poly urea is better because it's got a lot more hydrogen bond- ing. The hydrogen bonds are weak com- pared to covalent bonds, so they break f irst. That'll alleviate stress, and maybe your covalent bonds, the polymer chains themselves, remain intact." He adds that the beauty of hydrogen bonds is they will reform, so the material hasn't sustained

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